On the role of electrostatics in the heterolytic splitting of covalent bonds at defective oxide surfaces

A model is proposed for the heterolytic dissociation of covalent bonds at t he surface of perfect or defective oxide surfaces essentially based on the analysis of the electrostatic potential and its gradient at the bare surfac e. It permits us to obtain in a semiquantitative way the equilibrium geomet ry of the fragments and the reaction energy, and gives clues for the identi fication of the reaction path and for a rough estimate of the related activ ation energy. The predictive ability of the model has been verified by perf orming a number of calculations to simulate H-2 dissociation at various def ects at MgO and CaO: the isolated oxygen vacancy, the divacancy at the (100 ) face, the infinite edge and the divacancy at the edge. All calculations h ave been performed at an ab initio Hartree-Fock level of approximation, usi ng the CRYSTAL program for the periodic structures and the EMBED program fo r the local defects. Generally satisfactory agreement is found between the model predictions and the results of actual calculations. The model could b e useful for predicting with low cost computations if and how a local struc ture at a defective oxide surface is capable or not of heterolytically diss ociating strong covalent bonds. (C) 2001 American Institute of Physics.